As an emission type electronic displaying device, there is an electroluminescent device (ELD). As devices constituting the ELD, there are mentioned an inorganic electroluminescent device and an organic electroluminescent device (hereinafter also referred to as organic EL device). The inorganic electroluminescent device has been used for a plane-shaped light source, but a high voltage alternating current has been required to drive the device.
An organic EL device has a structure in which a light emission layer containing a light emission compound is arranged between a cathode and an anode, and an electron and a hole were injected into the light emission layer and recombined to form an exciton. The device emits light, utilizing light (fluorescent light or phosphorescent light) generated by inactivation of the exciton, and the device can emit light by applying a relatively low voltage of from several volts to several decade volts. The device has a wide viewing angle and a high visuality since the device is of self light emission type. Further, the device is a thin, complete solid device, and therefore, the device is noted from the viewpoint of space saving and portability.
An organic EL device for practical use is required which efficiently emits light with high luminance at a lower power. For example, there are disclosed a device with long lifetime emitting light with high luminance in which stilbene derivatives, distyrylarylene derivatives or tristyrylarylene derivatives are doped with a slight amount of a fluorescent compound (Patent document 1 below), a device which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a slight amount of a fluorescent compound (Patent document 2 below), and a device which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a quinacridone type dye (Patent document 3 below).
When light emitted through excited singlet state is used in the device disclosed in the above Patent documents, the upper limit of the external quantum efficiency (ηext) is considered to be at most 5%, as the generation ratio of singlet excited species to triplet excited species is 1:3, that is, the generation probability of excited species capable of emitting light is 25%, and further, external light emission efficiency is 20%.
Since an organic EL device, employing phosphorescence through the excited triplet, was reported by Prinston University (for example, see non-patent document 1 below), study on materials emitting phosphorescence at room temperature has been actively made (for example, see Non-patent document 2 or Patent document 4 below). As the upper limit of the internal quantum efficiency of the excited triplet is 100%, the light emission efficiency of the exited triplet is theoretically four times that of the excited singlet. Accordingly, light emission employing the excited triplet exhibits the same performance as a cold cathode tube, and can be applied to illumination. For example, many kinds of heavy metal complexes such as iridium complexes has been synthesized and studied (for example, see Non-patent document 3 below).
An example employing tris(2-phenylpyridine)iridium as a dopant has been studied (for example, see Non-patent document 2 below). Further, an example employing as a dopant L2Ir (acac) (in which L represents a bidentate ligand, and “acac represents acetyl acetone) such as (ppy)2Ir (acac) (for example, see Non-patent document 4 below), or employing as a dopant tris(2-p-tolylpyridine)iridium {Ir(ptpy)3}, tris (benzo-[h]-quinoline)iridium {Ir(bzq)3}, or Ir(bzq)2ClP (Bu)3 has been studied (for example, see Non-patent document 5 below).
A hole transporting material is used as a host of a phosphorescent compound in order to increase emission efficiency (for example, see Non-patent document 6 below).
Various kinds of electron transporting materials are used as a host of a phosphorescent compound, and further doped with a new iridium complex (for example, see Non-patent document 4 below). High emission efficiency is obtained by incorporation of a hole blocking layer (for example, see Non-patent document 5 below).
Also disclosed are compounds as thermally stable emission materials having a partial structure of a heterocyclic ring-containing polycyclic aromatic compound (see for example, Patent documents 5 and 6) and compounds as thermally stable hole transporting materials having a partial structure of a heterocyclic ring-containing polycyclic aromatic compound (see for example, Patent document 7). There is, however, no disclosure in these documents of a phosphorescence emission organic EL device.
Further disclosed are compounds having a fluorene joint (see for example, Patent document 8) as a thermally stable host materials), and compounds having a carbazole joint (see for example, Patent document 9). There is, however, no disclosure in these documents of a phosphorescence emission organic EL device.
A combination of a polycyclic aromatic compound and a phosphorescent compound is disclosed (see for example, Patent document 10), but it is not sufficient in view of emission efficiency or lifetime.
At present, an organic electroluminescent device emitting phosphorescence with further higher emission efficiency and longer lifetime has been studied. An external qauntum efficiency of around 20%, which is a theoretical threshold, is attained in green light emission, but in a low current region (a low luminance region), and the theoretical threshold is not attained in a high current region (a high luminance region). Further, a sufficient emission efficiency is not attained in another color emission, where there is room to be improved. An organic EL device for practical use is required which efficiently emits light with high luminance at a lower power. Particularly, an organic EL device is required which emits a blue phosphorescence with high efficiency.    Patent document 1: Japanese Patent No. 3093796    Patent document 2: Japanese Patent O.P.I. Publication No. 63-264692    Patent document 3: Japanese Patent O.P.I. Publication No. 3-255190    Patent document 4: U.S. Pat. No. 6,097,147    Patent document 5: Japanese Patent O.P.I. Publication No. 5-109485    Patent document 6: Japanese Patent O.P.I. Publication No. 7-53950    Patent document 7: Japanese Patent O.P.I. Publication No. 2001-43979    Patent document 8: Japanese Patent O.P.I. Publication No. 2000-30275    Patent document 9: Japanese Patent O.P.I. Publication No. 10-226785    Patent document 10: Japanese Patent O.P.I. Publication No. 2003-261471    Non-patent document 1: M. A. Baldo et al., Nature, 395, p. 151-154 (1998)    Non-patent document 2: M. A. Baldo et al., Nature, 403, 17, p. 750-753 (2000)    Non-patent document 3: S. Lamansky et al., J. Am. Chem. Soc., 123, 4304 (2001)).    Non-patent document 4: M. E. Tompson et. al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL′ 00, Hamamatsu)    Non-patent document 5: Moon-Jae Youn. Og, Tetsuo Tsutsui et. al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL′ 00, Hamamatsu).    Non-patent document 6: Ikai et. al., The 10th International Workshop on Inorganic and Organic Electroluminescence (EL′ 00, Hamamatsu).